5605
Organic and Biological Chemistry The Electronic Properties of Three-Membered Rings. I. uI and uR0Substituent Constants from Flg Nuclear Magnetic Resonance’ R. G . Pews2 Contribution from the Process Research Division, Esso Research and Engineering Company, Linden, New Jersey. Received February 16,1967 Abstract: The F19 nmr spectra of m- and pfluorophenylcyclopropanes, oxiranes, thiiranes, ethylenimines, and oxaziridines have been determined in dilute carbon tetrachloride solution. The inductive and resonance substituent constants have been calculated from the precise linear relationships developed by Taft and co-workers. The results of the present work are compared with the reported constants for structurally similar acyclic substituents and with the donor-acceptor properties of three-membered rings in the excited state.
T
he electronic properties of three-membered rings have received considerable attention in recent years. Since Walsh3 concluded from molecular orbital calculations and from ultraviolet spectroscopy that cyclopropane and oxirane possessed “properties of unsaturation,” many extensive studies have been carried out to determine if a three-membered ring can transmit or extend conjugation. Although most investigations have been directed toward an understanding of the cyclopropyl system, the electronic effects of the cyclopropyl ring are still a controversial issue. For example, Hammett p values are ambiguous over the ability of the cyclopropane ring to transmit conjugation. 4 , 5 This is also true for results obtained from ultraviolet spectroscopy.6-* Recently, Closs and Klingerg have concluded from the analysis of the H1 nmr spectrum of phenylcyclopane that there is an electronic interaction between the cyclopropyl and aromatic rings. The electronic interaction of the oxirane or the aziridine ring with an adjacent carbonyl group has been reported by Cromwell and co-workers. The results of their investigation showed that the three-membered ring in trans-arylaroylethylenimines or oxides can transmit the electrical effect of a P-phenyl group to the a-carbonyl as measured by the ultraviolet absorption spectra. More recently Ketcham, et a1.,12 have shown (1) Presented before the Division of Organic Chemistry, 153rd National Meeting of the American Chemical Society, Miami Beach, Fla., April 1966. (2) Inquiries may be addressed to the Dow Chemical Co., Midland, Mich. 48640. (3) A. D. Walsh, Trans. Faraday SOC.,45, 179 (1949). (4) E. N. Trachtenberg and G. Odian, J . A m . Chem. Soc., 80, 4018 (1958). (5) R. Fuchs, C. A. Kaplan, J. J. Bloomfield, and L. F. Hatch, J . Org. Chem., 27,733 (1962). (6) G. W. Cannon, A. A. Santilli, and P. Shenian, J . Am. Chem. SOC., 81, 1660 (1959). (7) R. H. Eastman and S . K. Freeman, ibid., 77,6642 (1955). (8) R . J. Mohrbacher and N. H. Cromwell, ibid., 79, 401 (1957). (9) G. L. Closs and H. B. Klinger, ibid., 87, 3265 (1965). (10) N. W. Cromwell, R. E. Bambury, and J. L. Adelfang, ibid., 82, 4241 (1960). (1 1) N. H. Cromwell, F. H. Schumacher, and J. L. Adelfang, ibid., 83, 974 (1961). (12) L. A. Strait, P. Ketcham, D. Jambotkar, and V. P. Shah, ibid., 86,4628 (1964).
that, in the excited state, oxirane and thiirane are electron withdrawing relative to benzene, whereas cyclopropane is electron donating. These results will be described further in the discussion part of this paper. In the present investigation, the ground-state donor-acceptor properties of three-membered rings have been determined relative to benzene by F19 nmr. A definitive answer to the ability of the cyclopropane, oxirane, thiirane, and aziridine rings t o transmit conjugation in the ground state will be presented in a future publication on the F19 nmr shielding parameters in stilbenetype systems.
Results and Discussion The extreme sensitivity of fiuorine shielding in mand p-fluorophenyl groups to small intramolecular perturbations produced by the ring substituents has been well established. 1 3 - 1 6 The precise linear relationships developed by Taft from the F19 nmr shielding parameters of meta- and para-substituted fluorobenzene permit the detection of and the separation of inductive and resonance effects. These correlations which are based on experimental work13-16 as well as theoretical justification 17, l 8 are presented in the following equations where .fgX and .@-x are the fluorine chemical shifts in parts per million ofpara- and meta-substituted fluorobenzenes, respectively, relative to a fluorobenzene standard, and uI and uR0 are the inductive and resonance substituent constants.
JLX -JZX
= -(29.5)0Ro
(13) R. W. Taft, E. Price, I. R . Fox, I. C. Lewis, K . I(. Andersen, and G. T. Davis, ibid., 85, 709, 3146 (1963). (14) R. W. Taft, Jr., S. Ehrenson, I. C. Lewis, and R. E. Glick, ibid., 81, 5132 (1959). (15) R. W. Taft, Jr., ibid., 79, 1045 (1957). (16) R. W. Taft, Jr.,J. Phys. Chem., 64, 1805 (1960). (17) F. Prosser and L. Goodman, J . Chem. Phys., 38,373 (1963). (18) R. W. Taft, Jr., F. Prosser, L. Goodman, and G. T. Davis, ibid., 38,380 (1963).
Pews /
UX
and U R O Substituent Constantsfrom FIBNmr
5606
The high reactivity of oxiranes, thiiranes, and aziridines with acidic reagentslg excludes any possibility for the determination of the inductive and resonance substituents constant in the usual manner.20 F19 nnir is, therefore, a unique spectroscopic probe for such an investigation. The chemical shifts and calculated substituent constants for the cyclopropane, oxirane, thiirane, aziridine, and oxaziridine are recorded in Table I. For purposes
carboxylic, and P-phenylpropionic acids, that the cyclopropane ring does not transmit conjugation. * ? This equivalence of electronic properties for the cyclopropyl and methyl groups in uncharged fluorobenzenes is noteworthy when compared with the results for the effect of the same substituents in the stabilization of carbonium ions. Taft and TsunoZ3have found that the cyclopropyl and methyl substituents were decidedly different in their ability to stabilize the carbonium ion system
Table I. Shielding Parameters and Substituent Constants of Three-Membered Rings
1.18
-0.08
5.11
-0.13
0.96 1.18 0.65
-0.05
-0.08 0.01
4.94 5.40 1.40
-0.13 -0.15 -0,03
0.11
0.07
1.43
-0.04
0.10
0.07
1.80
-0.06
0.25 0.14
11.50 4.30
-0.43 -0.17
3.26
-0.085
-1.05 -0.38 0.75
-0.02
0.10
0.07
8.52
-0.29
0.58 0.50
0.00 0.01
3.60 14.20
-0.15 -0.48
0.13 -0.18
0.07 0.11
-1.55 -3.10
0.05 0.11
V. F. Bystrov, 0. A. Yuzhakova, and R. G. Kostyanovskii, Dokl. Akud. Nuuk SSSR, 147, 843 (1962).
of comparison, values of uI and U ~ Oas, determined by FI9nmr, have been included for substituent groups with similar structural features, The selection of a saturated aliphatic substituent for comparison with the cyclopropyl group can only approximate in structure the components of the three-membered ring since no saturated aliphatic substituent is comprised of three carboncarbon and five carbon-hydrogen bonds. Fortunately, precise values of uI and uR have been determined from reactivity correlations for methyl, ethyl, isopropyl, and t-butyl and were found to differ by an average of only i0.015 in both uI and uR, respectively.?l The results from Fl9nmr, which are summarized in Table I, show that the inductive and resonance substituent constants for the alkyl and vinyl groups are sufficiently different so that a definitive comparison can be made with the cyclopropyl group. The calculated values of uI and uR0 for the cyclopropyl group are -0.08 and -0.13, respectively, and thus are essentially the same as for the alkyl substituents. The present results provide strong support for the findings of Trachtonberg and Odian4 who concluded, from a comparison of the Hammett p values for substituted trans-cinnamic, trans-2-phenylcyclopropane(19) A . Weissberger, Ed., “Heterocyclic Compounds,” Vol. 19, Interscience Publishers, Iiic., New York, N. Y . , 1964. (20) L. P. Hammett, “Physical Organic Chemistry,” McGraw-Hill Book Co., Inc., New York, N . Y., 1940. (21) R. W. Taft, Jr., and I. C. Lewis, Tetrahedron, 5,210 (1959).
Journal of the American Chemicul Society
The values, in parts per million, which were obtained for the mesomeric charge p e r t ~ r b a t i o n , ? ~ are as follows: R = methyl = - 27.5; R = cyclopropyl = -21.4; R = CH=CHC6H5 = - 19.9. Similar results have been reported in the solvolysis of para-substituted phenyldimethylcarbinyl ~ h l o r i d e s . In ~ ~this ~~~ system, the cyclopropyl group is more effective than the isopropyl in stabilizing the electron-deficient center as evidenced by the rate enhancement and decrease in AH* for the cyclopropyl when compared to the isopropyl substituent, It is clear from the above discussion that the substituent effect of the cyclopropyl group is strongly dependent on the electronic demand of the system. Furthermore, the stronger dependence of the substituent effect of the cyclopropyl group on its electronic environment affords an explanation for the larger pK,+ value for the tricyclopropyl cation then is observed for either the trityl or t-butyl cations. 27 Oxirane and thiirane exhibit the same inductive effect in the meta position. The sign of uI for these substituents is in agreement with that predicted on the basis of sign of the m-OCH3 and m-SCH, groups. However, the magnitude of the inductive effect is considerably smaller than either m-SCH3 or m-OCH,. This decrease in electron-withdrawing ability may be attributed to the interposition of the methylene group between the heteroatom and the aromatic ring. The attenuation of the inductive effect by one methylene group has been calculated from Taft’s u* values for the following pairs of substituents ClCH2CH2/CICH,, CF3CH2CH2CH&F3CH2CH2, and C6H5CH2CH2/C6H5CH2.The resultsz8 were 0.37, 0.27, and 0.37 or an average of 0.34. McG ~ w a n has ? ~ concluded from a discussion of various methods of estimating the attenuation of the inductive effect per CHa- group that a value of 0.50
si:$,
(22) Cromwell and Mohrbachers concluded that the cyclopropyl ring can transmit conjugation in the excited state in 1-aroyl-2-arylcyclopropanes. Apparently, transmission of the phenyl ring unsaturation through the cyclopropane ring into the aroyl chromophore occurs, causing a smaller energy difference between the ground and the excited states of the 2-phenylcyclopropyl ketones as compared with the analogous energy difference in the unsubstituted cyclopropyl ketones. (23) R. W. Taft and T. Tsuno, unpublished experiments. We wish to thank Professor Taft for making these results available to US. (24) The mesomeric charge perturbation (mcp) = (aPF - 6,,”), where AmF refers to the F ’ g nmr shielding parameter of a meta-substituted fluorobenzene and ?iPFrefers to that for the corresponding para isomer. ( 2 5 ) L. B. Jones and U. K. Jones, Terrahedron Letters, 1493 (1966). (26) H. C . Brown and J. D. Cleveland, J . A m . Chem. Soc., 88, 2051 (1966). (27) N. Deno, H. G. Richey, Jr., J. S. Liu, D. N. Lincoln, and J. 0 . Turner, ibid., 87,4533 (1965). (28) J. E. Leffler and E. Grunwald, Ed., “Rates and Equilibria of Organic Reactions,” John Wiley and Sons, Inc,, New York, N. Y., 1963, Chapter 7 . (29) J. C. McGowan, J . Appl. Chem. (London), 10, 312 (1960).
89:22 J October 25, 1967
si:$
5607
gives better agreement in many cases. Therefore, the values of uI for oxirane and thiirane are in good agreement with theory. The direction of the resonance effect of the oxirane and thiirane substituent is the same as for the p-OCH3 and p-SCH3 groups. Direct conjugation of the heteroatom with the aromatic ring is inhibited by the methylene group and the magnitude of uR0 is reduced. The magnitude of uRo for the threemembered rings appears to be directly related to the electronegativity effect of the heteroatom for the firstrow elements, i.e.
of the two heteroatoms. It should also be noted that the oxaziridines are isomeric with the monomethylamides. Although the uI and uR0 values are not available for the monomethylamide, the electron-withdrawing property of the amide substituent in both meta- and para-substituted benzenes is well documented. l 3 The opposite sign of uR0 for the oxirane and the oxaziridine indicates that the oxygen bridge between carbons permits very little, if any, contribution from resonance structure I, whereas structure I1 must make a measurable
H
I
L>A>_a The nature of this electronegativity effect on uR0 is the result of an “internal inductive” effect of the heteroatom on the carbon CY to the aromatic ring. Since oxygen is more electronegative than carbon, the uR0 value of -0.04 for oxirane is less negative than the uR0 value of - 0.13 for the cyclopropyl group. Although sulfur and carbon have the same electronegativity, the uR0 value for the thiirane substituent is considerably less negative than that for the cyclopropyl, group. This may indicate that a favorable geometry exists for the interaction of system of the arothe d electrons on sulfur with the i~ matic ring. The electronic properties of aziridines are of special interest in that the heteroatom may be attached directly, CY, or one carbon removed, /3, to the aromatic ring. The results in Table I show a change in the sign of the inductive substituent constant, uI, with a change in position of the nitrogen in the aziridine ring. This change of sign for uI for the aziridines is due to the increased effect of the more electronegative nitrogen, compared to carbon, when placed in a position CY to the aromatic ring. In the series
I
contribution to the resonance parameter of the oxaziridine. A comparison of the donor or acceptor properties in the ground state of the three-membered rings with the results obtained from ultraviolet spectroscopy (excited state) is instructive. From the ultraviolet spectra of a series of para-substituted phenylcyclopropanes, styrene oxides, and styrene sulfides, Ketcham, et ale,’* showed that, relative to benzene, oxirane and thiirane are electron withdrawing, whereas cyclopropane is electron donating. The change of oxirane and thiirane from donor to acceptor species in going from the ground state to the excited state may be attributed, firstly, to a large contribution of the quininoid structures I11 and IV in the excited state, and secondly, to the absence of any such contribution in the ground state. Since substituent constants are determined in the ground state, the inver-
X~ e ( ; . - c \H ~ I11 X = O or S
H I
A
N
A
-1173
becomes increasingly positive (see Table I). The trend in this series is in good agreement with the previous observation that the first atom of the substituent plays a predominant role in determining the uI parameter for the substituent as a whole. l 3 The resonance parameter uREis larger for the N-arylaziridine than for the isomeric 2-arylaziridine. This is in agreement with the larger value of p-fluoroaniline when compared with p-fluorobenzylamine. The equality of the ratios of the following uR0 parameters indicates that the aziridine crI
H
ring does not posses any donor-acceptor properties akin to an olefinic group, The introduction of two heteroatoms into the threemembered ring to form an oxaziridine produces an electron-withdrawing effect in both the meta and para positions. The value of uI for the oxaziridine is more positive than any of the above-mentioned three-membered rings with the combined electronegativity effect
I1
I?
+e 7.. -
%H,
H IV
sion of the donor-acceptor properties of oxirane and thiirane provide some explanation for the lack of any really good relationship between AA and any parameter measuring the resonance effect of a substituent.30 Experimental Section Materials. The m- and p-fluorostyrene oxides were prepared by the epoxidation of the corresponding olefins with m-chloroperThe oxaziridines were prepared by oxidation of the benzoic acid. corresponding Schiff bases with m-chloroperbenzoic acid. The styrene sulfides were prepared from the oxides by the procedure ~ m- and p-fluorostyrenimines were predescribed by G u ~ s . 3 The pared by the Wenker synthesis by the procedure described by B r o i ~ . The ~ ~ m- and p-fluorocyclopropanes were prepared from olefin cia the intermediate gem-dibromocyclopropanes as described by Ketcham.34 In addition to the elemental walysis recorded in Table 11, the compounds were further characterized by their proton nuclear (30) H. H. Jaff6 and M. Orchin, “Theory and Applications of Ultraviolet Spectroscopy,” John Wiley and Sons, Inc., New York, N. Y., 1962, p 259. (31) N. N. Schwartz and J. H. Blumbergs, J . O r g . Chem., 29, 1976 (1964). (32) C. 0. Cuss and D. L. Chamberlain, Jr., J . A m . Chem. SOC.,74, 1342(1952). (33) S.J. Brois,J. O r g . Chem., 27, 3532 (1962). (34) R. Ketcham, R. Cavestri, and D. Jambotkar, ibid., 28, 2139 (1963).
Pews
UI and UIXO
Substituent Constants from FlyNmr
5608 Table I1 Compound
m-FC6H4C-NCH3a a
Formula
Bp, " C ( m m )
CsHsFON
@D
-C, Calcd
%Found
-H, Calcd
%Found
--F, Calcd
%Found
-S
or N, %-Calcd Found
90(45)
1.5051 79.38
79.71
6.66
7.00
13.95
13.30
85(40)
1.5049 79.38
79.84
6.66
6.71
13.95
13.0
92(14)
1.5074 69.56
69.94
5.11
4.68
13,75 13.5
89(13)
1.5078 69.56 69.95
5.11
5.41
13.75
52-56(0.5)
1.5610 62.31
63.08
4.58
4.97
12.32 12.5
20.75
19.47
6O-65(1-2)
1.5601 62.31
63.02
4.58
4.72
12.32
13.4
20.75
19.10
132-1 38(6-8)
1.5310 70.05
69.38
13.85
14.5
10.22
10.15
127-129(6)
1.5330 70.05
69.93
13.85
14.0
10.22 9.81
45(0,05)
1.5080 62.74 62.85
5.24
4.98
4X0.05)
1.5076 62.74
5.24
5.41
62.24
13.3
Active oxygen by iodometric analysis was >90%.
magnetic resonance spectrum. The chemical shift of the aliphatic protons were found in every case to be identical with that observed in the spectrum of the corresponding nonfluorinated compound. Procedure. Fluorine nmr spectra were obtained with Varian Associates high-resolution nmr spectrometer operating at 56.4
Mcps. Approximately 5 (volume) solutions of the fluorobenzenes were used with either 1,1,2,2-tetrachloro-3,3,4,4-tetrafluorocyclobutane or o-difluorobenzene as the internal reference. The chemical shifts were then related to fluorobenzene as described previously.13
Transfer Reactions Involving Boron. XIV. The Stereochemistry of a-Transfer Reactions' Daniel J. Pasto and James Hickman2 Contribution f r o m the Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556. Received April 21, 1967 Abstract: T h e stereochemistry of the rearrangement of a-chloroorganoboranes, formed in p a r t i n the hydroboration of vinyl chlorides, to alkylchloroboranes is shown t o proceed with complete inversion of stereochemistry a t carbon.
T W
he addition of borane to a heterosubstituted olefin results in the formation of both a-and P-heterosubstituted o r g a n o b o r a n e s (2 and 3, respectively), the r e l a t i v e amounts of the two isomers depending on the heterofunctional group X. Previous investigations in 1 2 3 our l a b o r a t o r i e s have shown that 2 undergoes spontaneous rearrangement to 4,3-5 termed an a t r a n ~ f e r , ~ that this rearrangement occurs prior to the basic oxidation-hydrolysis work-up procedure, and that 2 does (1) Part XIII: D. J. Pasto, J. Chow, and S. I
